Circular economy (CE) is one of the key concepts to improve sustainability through reducing resource dependency and scarcity but increasing resource efficiency. Currently there are no holistic methods to analyze and assess product and material cycles systematically, which would however be a prerequisite to further improve CE. Thus, this contribution presents a methodology for assessing the circularity of products along their whole life cycle. In this way, the greatest potentials for improving circularity can be identified and then iteratively optimized. The methodology is founded on a bottom-up approach and consists of three phases. At the beginning in the data collection phase, all necessary data is gathered and stored in a digital product passport. The key data needed for the assessment are material data (e.g., origin, recycled or primary), energy used, working time from humans and machines, transport routes and involved processes with required tools. In the following evaluation phase, various circularity indicators and a detailed profile of the evaluated product are created. The results are interpreted in the third phase and requirements are derived from the product profile with the indicators to improve the circularity of the product. The methodology is demonstrated on the use case of a spice grinder.